The present invention relates to a method for obtaining a tomographic image of a body to be examined, using nuclear magnetic resonance (hereinbelow called NMR) phenomena and in particular to a method for correcting deviations in the phase of NMR signals produced by variations in the static magnetic field, capable of improving the image quality, and a device for realizing same.
For the method for generating the static magnetic field in an MRI device there are three systems, i.e. superconductive magnet system, resistive magnet system and permanent magnet system. Fluctuations in the static magnetic field in a device using the superconductivity are in an order of 0.1 ppm/h and thus a static magnetic field extremely stable can be obtained. In the resistive magnetic system an extremely stable current source is required. However, the static magnetic field obtained thereby has fluctuations of an order of several ppm due to short time drift in a driving circuit therefor or mixing of hum as well as temperature fluctuations in cooling water for a coil. In the permanent magnet system the material itself has a temperature coefficient having an order of magnitude of 200 to 2000 ppm/C.degree., although it varies, depending on the material and therefor small temperature variations can be a factor of fluctuations in the static magnetic field.
The fundamental principle of the imaging method using the NMR phenomena is that the resonance frequency thereof is proportional to the intensity of the magnetic field. Consequently, when the intensity of the static magnetic field severing as the base for the method is varied, the frequency of the obtained signal is also varied, which manifests itself in the form of deviations in the position of the image and produces ghost and blur in the image, causing lowering the image quality.
Therefore, in an MRI device by the prior art resistive magnet system, magnetic field locking means is used often for correcting the intensity of the static magnetic field by measuring directly or indirectly the intensity of the static magnetic field before or during the measurement to feed it back. However, in the permanent magnet system, since it has neither coil nor driving device producing the static magnetic field, it causes elevation in the cost and prolongation of measurement time to dispose the magnetic field locking means therefor.
As a prior art technique for dealing with variations in the static magnetic field in the MRI device using a permanent magnet, there is known that disclosed in JP-A-61-280551. However, by this correcting method, since correction is effected in unit of pixel after Fourier transformation, no attention is paid to deviations within one pixel. Since even deviations within one pixel can cause fairly remarkable blur from the point of view of the image quality attention should be paid thereto.